1. Field of Invention
This invention relates to biomass gasifiers, more particularly to downdraft-channel gasifiers for the efficient conversion of biomass materials to usable heat energy or fuel for internal combustion engines.
2. Description of the Prior Art
The depletion of our non-renewable petroleum energy resources has caused increased interest in renewable biomass energy resources. The potential energy contained in crop residues, wood residues, and biomass crops all present attractive possibilities for reducing our dependence on fossil fuels.
Modern agricultural industry has a particular dependence upon petroleum energy. Not only are huge quantities of petroleum fuels consumed by equipment used in field work, but drying corn can require twice the heat energy of the liquid fuel required for crop production operations, including harvesting. Therefore, there is a real need for a furnace to efficiently burn biomass and produce heat for drying farm crops. Drying corn with corn cobs represents a major opportunity for reducing the dependence of Corn Belt farmers on fossil fuels, since the heat from one cob can dry the grain from at least three ears of corn.
The efficient conversion of biomass materials to usable heat energy requires a different technology than that developed for petroleum products and coal. The major differences are as follows:
1. Usually the biomass material is not free-flowing, and is often not in uniform discreet particles, thus it requires complex handling and feeding equipment. PA1 2. Biomass material contains a high proportion of volatile material (up to 80%) which vaporizes very rapidly and can cause much smoke and loss of energy if combustion is not controlled and completed. PA1 3. Many biomass materials contain silica and the ash fuses into an adherent slag at temperatures above 800.degree. C. (1472.degree. F.). PA1 4. Biomass material has a high moisture content as grown and requires natural or artificial drying before use as a fuel. PA1 1. Minimum air pollution and direct-drying of the product without the necessity of using a heat exchanger which reduces efficiency and adds equipment expense. PA1 2. More efficient conversion of biomass to heat, with 80-90% of its heat recovered. PA1 3. Control of combustion rate by regulating primary air flow. PA1 1. Tars are burned before they can condense. PA1 2. The design does not require that combustion air pass through the biomass fuel. Any combustible biomass material which will feed down in a straight-sided chamber and will also be primarily supported on the air ducts can be used. PA1 3. The fuel in the gasifier chamber burns almost completely to ashes with little char remaining.
Direct combustion of biomass materials usually results in smoke and ash pollution unless special filtering equipment is used. Smoke from wood-fired household stoves or fireplaces may cause tar (creosote) deposits in the chimney which may eventually ignite, emit dangerous sparks, overheat chimneys and cause fires. Heat exchangers are often needed in order to avoid contamination of the area or the material being heated.
In attempts to solve many of the problems associated with direct combustion, devices have been built which gasify biomass materials to produce a combustible gas which could in turn be used for heating purposes. Gasification is the process whereby biomass is burned with limited air (gasified) to produce an exhaust gas (producer gas) which contains enough CO, H.sub.2 and some CH.sub.4 to be combustible. This gas is drawn off to a secondary combustion chamber where heat energy is produced with little or no smoke or ash or it can be used to fuel internal combustion engines.
One comparative drawback is that this gas's heating value is only 3.72-7.45 MJ/m.sub.3 (100-200 Btu/ft.sub.3) compared to 37.2 MJ/m.sub.3 (1000 btu/ft.sub.3) for natural gas. Producer gas is diluted by the inert nitrogen in the combustion air, by the CO.sub.2 produced during combustion, and by water vapor; hence, its low heat value.
The gasification process has been in use for a considerable period of time. The producer gas derived from the gasification of coal was in common use before natural gas became available; but as stated before, conversion of biomass materials to usable heat energy (including gasification) differs markedly from methods developed for petroleum products and coal.
The major experience with gasification occurred during World War II when Sweden and other northern European countries could not obtain petroleum. Portable gasifiers using charcoal or wood chips were used to produce gas to power motor vehicles and tractors, although with greatly reduced power and convenience.
During that time, it was also found that a special type of gasifier, called a downdraft gasifier, could produce a gas from wood chips usable by motor vehicles, whereas the more widely known updraft gasifiers required charcoal fuel to avoid excessive tar. About half of the heating value of wood is lost in making a charcoal, so direct use of wood chips is much more efficient.
Gasifiers are usually classified as being updraft, downdraft, or sidedraft (crossdraft). With updraft gasifiers, air is blown or drawn up through a grate supporting the biomass, and this producer gas passes up through the combustion zone and the unburned fuel and out an exit at the top for use. Downdraft gasifiers, on the other hand, force the producer gas down through the combustion area by means of air pressure or air suction to an exit for use.
Conventional gasification of biomass is attractive for crop drying because it is possible to burn the producer gas like natural or LP gas without visible smoke or particulates. Clean combustion will allow the flue gas to be mixed with ambient air and used for drying of crops, as with natural or LP gas. However, if the producer gas cools as it is piped from the gasifier to the burner-dryer fan assembly, any tars may condense in the pipe. Possible advantages of gasification over direct combustion of biomass for drying crops are:
Research and development of large-scale gasifiers for crop drying is being attempted. In particular, updraft gasifiers of up to 9488 MJ/hr (9 million Btu/hr) capacity are being developed by seed corn companies to utilize their by-product corn cobs as a fuel for drying their ear corn.
However, updraft gasifiers present significant problems when used as suppliers of gas for drying of corn. Although sufficient air may be supplied to promote gasification, high oxidation temperatures create slagging problems. Also, the gases which are produced do not pass through a cracking zone and are difficult to burn cleanly without tar. Finally, incomplete combustion creates exhaust by-products which may discolor and impose an unfavorable and detrimental odor to the corn which is being dried.
Downdraft gasifiers were developed in an effort to reduce tar content in gas used to power internal combustion engines. In conventional downdraft systems, the air enters through peripheral jets directed toward the center of the airtight cylindrical fuel and combustion chamber just above a funnel-shaped or conical bottom to initiate gasification. As the biomass is devolatilized by combustion and heat, it shrinks into pieces of carbon or char which accumulate in a conical bottom and are supported on a grate below the throat. The char forms a red-hot bed which reduces CO.sub.2 to combustible CO and also cracks the tars into stable gases. These can then be cleaned and cooled for use by an engine. Problems with this type of gasifier are primarily associated with the fact that a conical bottom downdraft gasifier does not scale up efficiently for high gas generating capacity, such as needed for drying corn with corn cobs. This is so because as the size of the conical or funnel-shaped bottom where the combustion air enters is increased, the peripherally-introduced air has more difficulty in penetrating the fuel mass uniformly and producing a uniform char bed in the bottom of the cone. It is the passing of the producer gas through glowing char which cracks the gas into shorter chain hydrocarbons. Therefore, less uniform cracking of the hydrocarbons will be accomplished; and the producer gas will not be as clean as desired. Combustion with limited air to produce CO instead of CO.sub.2, is not so much a matter of limiting the air as it is of distributing the available air throughout the fuel mass to contact the greatest possible fuel surface area.
A new concept, called channel gasification, was devised by this applicant in an attempt to obtain the low-tar producer gas of the downdraft type of gasifier, but having a large bed area, uniformly supplied with air, as obtainable with a high-capacity updraft type. A large gasifier of this type might permit the gasification of hay, straw, and corn fodder in large, round bales or small stacks, as well as biomass in discreet particles such as corncobs or wood chips.
In the applicant's original channel gasifier prototypes, the biomass material rested on a corrugated floor of a chamber with a tight cover with the material supported primarily on inverted V-shaped or triangular air ducts, leaving open or air-permeable V-shaped channels defined by the air ducts. Combustion air flowed into the channels from holes along the air duct top edges. Burning of the biomass material took place in the channels and the material moved down by gravity as burning proceeded. Producer gas traveled longitudinally in the channels to end outlets.
The primary virtue of the channel concept is its capability for scaling up without losing efficiency, along with its simplicity, its cooling capability and its adaptation to fabrication from steel sheets. Therefore, downdraft channel gasification offers the potential of overcoming the problem inherent in the previously downdraft conical bottom gasifiers, thus allowing scale-up with efficiency. Corncobs, chopped corn stover, wheat straw, sawdust, wood chips from prunings, and wood scraps can be successfully burned with little or no smoke.
Possible advantages of channel downdraft gasification with close-coupled gas combustion over conventional updraft gasifiers are:
The applicant's original channel gasifiers were constructed to be of the cross-draft type. Air was introduced from holes in the ridge tops and gas flowed longitudinally in the channels to end outlets. However, problems were found with this system. The gases produced in the cross-draft gasifier above the channel near the exit passed through very little glowing char compared to gases originating at the opposite end, giving inadequate cracking action.
It is therefore an object of this invention to provide a downdraft channel gasifier which provides for efficient gasification and uniform cracking action of the long-chain hydrocarbons inherent in the producer gas.
A further object of this invention is to provide a downdraft channel gasifier which presents multiple channels to which are supplied uniform primary air creating a uniform bed of combusting biomass for efficient gasification and cracking of the long-chain hydrocarbons inherent in the producer gas.
A further object of this invention is to provide a downdraft channel gasifier which provides for the efficient conversion of biomass materials to usable heat energy or fuel for internal combustion engines.
Another object of this invention is to provide a downdraft channel gasifier which produces lower combustion temperatures compared to direct combustion or updraft gasification, reducing the cost of furnace materials and also reducing the formation of slag which can block air openings, etc.
Another object of this invention is to provide a downdraft channel gasifier which reduces the tar content in the producer gas.
Another object of this invention is to provide a downdraft channel gasifier which takes advantage of the bridging characteristics of most biomass fuels by supporting them on triangular ducts which also furnish combustion air which cools the duct material as the air is preheated.
Yet another object of this invention is to provide a downdraft channel gasifier which results in hotter and cleaner burning gas.
Additional object, features, and advantages of the invention will become apparent with reference to the accompanying specification and drawings.